Project Summary In radiation therapy, cancer tumors are targeted with ionizing radiation that induces cell death by damaging DNA. Radiation affects malignant and healthy tissue, and treatment plans strive to deliver a lethal radiation dose to tumors while minimizing collateral damage to surrounding tissue. While standard therapy employs photon radiation, the clinical use of proton radiotherapy is increasing because of its ability to localize dose. Compared to photons, which display an exponential decrease in deposition with penetration depth, protons deposit a large fraction of their energy in the last few mm of their path due to the sharp distal falloff. The localized energy deposition peak is called the Bragg peak. Because of the peaked deposition profile, tissue before and, particularly, after the target region receive a lower relative dose compared to photon treatment. The penetration depth depends on the initial kinetic energy of the protons and the stopping power of the irradiated material. The main advantage of proton radiotherapy for treatment of cancer is the proton Bragg peak. Unlike photons and electrons, the finite range and the sharp dose falloff at the distal end of the proton beam's Bragg peak increases our ability to conform the treatment dose to the tumor and spare the surrounding healthy tissues (Knopf and Lomax, 2013). However, there are uncertainties in our ability to precisely locate the proton beam Bragg peak and its distal dose gradient within the patient, which often results in a deliberate over- and undershoot of the beam into healthy tissues located in front of and beyond the tumor. This substantial increase to the margins undermines the benefits of the proton's unique steep dose gradient, reducing the clinical potential of proton radiotherapy. To fully exploit the advantages of the proton Bragg peak, there is a critical need to reduce proton beam range uncertainties especially at the distal edge where a sharp dose gradient exists (Knopf and Lomax, 2013). PET imaging and prompt gamma techniques have been proposed and tested as in situ range verification techniques, but their lack of accuracy, complexity, and cost have motivated researchers to explore other methods. Protoacoustics, the measurement of sound waves generated by proton beams, is an undeveloped, potential in situ range verification technique.